|
Magnification is the reason people buy a pair of binoculars. Binoculars are typically described using a pair of numbers, such as 10x50. The first number refers to the binoculars' magnification power. In the above example, the binocular offers 10X magnification; that is, the image you focus on will appear 10 times closer or larger than it does to the unaided human eye. Common binocular magnifications are 6X, 7X, 8X,10X, and 20X. The second number ("50" in our example) refers to the diameter, in millimeters, of the front lens. The front lens is the objective lens, and generally, given two binoculars with the same magnification, the larger it is, the brighter the image will appear in low-light conditions.
The two most popular designs used to make modern binoculars are based on porro prisms and roof prisms. The prisms in binoculars correct the upside down and reversed image you would see without them. Binoculars using the porro prism design have a "bend" or "jog" in each barrel so that the objective lenses are offset from the eyepieces. The roof prism design allows each barrel to be straight.
Both prism types have advantages and disadvantages. Porro prism binoculars feature a system in which two separate prisms are positioned in an offset relationship so that there is greater separation between the objective lenses than the eyepiece lenses. They provide good optical quality, greater three-dimensional view, and are more affordable than roof prisms. However, porro prisms are also less compact, not as comfortable to hold for long periods of time, and may be less rugged than roof prisms.
Binoculars with a roof prism design are considerably more compact and are usually easier to hold for long periods. Roof prism binoculars feature a system in which prisms are positioned in an in-line assembly, resulting in a straight-barreled, more compact binocular. They are generally manufactured to closer optical and mechanical tolerances making them more rugged than porro prism binoculars, but they also come with a higher price tag.
Angle of view is the width of the area you see in your binoculars and is represented by the third number usually found on the binoculars. It is sometimes expressed as a number of degrees, such as six degrees or seven degrees, while other times it is listed as the field of view at 1,000 yards. The wider your angle of view the easier it will be to locate your subject, particularly if it is moving. High magnification binoculars have narrower fields of view than those with lower power, so in comparing fields of view, make sure you are looking at models with the same magnification. Binoculars with an apparent angle of view of 65 degrees or more are considered wide angle, regardless of their magnification power. However, some wide angle binoculars sacrifice eye relief.
Eye relief is a feature that allows comfortable viewing while wearing eyeglasses or sunglasses. Eye relief refers to the maximum distance at which you can place your eye from the lens to receive the widest field of view for the style of binoculars you own. Some binoculars come with long eye relief, allowing you to see the complete field of view with or without eyeglasses. However, users with astigmatism get the best results from their binoculars with their eyeglasses on.
Nearly all manufacturers put some type of coating on at least one of the optical surfaces in their binoculars. Coating a lens surface means more available light is transmitted to your eye. Glare is reduced and image contrast is enhanced.
The exit pupil diameter is a figure used to measure the brightness of an image as it's delivered through the binoculars' eyepiece. The larger the exit pupil, the brighter the image will be under low-light conditions. It can be seen as the bright circle that is visible when the ocular lens is viewed at a distance of about 25 cm with the objective lens facing towards a bright background or light source. To calculate the exit pupil, divide the objective lens size by the magnification power. For example, using an 8x40 binocular, your exit pupil is 5: 40mm / 8x = 5 (exit pupil size)
Binoculars come in a variety of designs and sizes depending on your application. Some applications, particularly birding and wildlife viewing, expose binoculars to harsh conditions such as rain, snow, and wind. To address this, many binoculars are available in water-resistant models that are designed to stand up to extreme climates and still perform well.
Full-size binoculars offer more powerful magnification levels which can be useful in many applications such as wildlife viewing and surveillance. The added weight, however, may make them uncomfortable to hold over long periods of time. For indoor applications, including theatre and sports viewing, compact and lightweight models are available that still deliver exceptional performance relative to their size.
|
